Nowadays fossil fuels, such as gas, oil and coal are largely used for producing electrical or heat energy, for burning away waste or for converting or purifying substances, in particular ore. Due to an increasing demand of electrical energy and the concurrent pullout from running nuclear power plants in countries such as Germany and Japan, coal has gained an increased attraction, since it is also present in industrial countries themselves.
Upon burning fossil fuels, in particular coal, carbon dioxide is released, but also other noxious substances, such as e.g. sulphur dioxide, nitrogen dioxide, carbohydrates, hydrochloric acid, dioxins, furans, fly-ash, carbon black, but also highly toxic heavy metals, such as e.g. mercury. Such problems especially arise in waste incinerating plants, wherein an inhomogeneous mixture of different disposed articles are burnt in conjunction with the fossil fuels, so that the toxins contained in the materials or produced during combustion thereof are emitted into the environment.
Among others for these reasons combustion plants are not allowed to be run without a complex purification system, which enables an essentially complete removal of toxic substances from the combustion gases. However, in spite of all of the measures taken, the amount of mercury in the environment increases, which increase is mainly due to an increasing use of coal in combustion plants.
According to information provided by the environmental program of the United Nations (UNEP) the amount or mercury (Hg) emitted into the environment in 2005 sums up to about 498 tons, of which about 6 tons emerge from German power plants, about 50 tons from US generating plants and more than 100 tons from Chinese power generating plants.
For purifying combustions gases dust removal processes are conventionally put to use, such as using surface filters, electric filters, gas cleaners, and centrifugal force separators. For removal of gaseous and fluid matter methods of thermic post combustion, of catalytic transformation or methods of adsorption and absorption are used. Due to economic aspects the purification of combustion gases focuses on the methods of adsorption and absorption, by means of which an effective reduction of toxins in said combustion gases may be achieved.
Absorption may be achieved simply my guiding the combustion gases through water, which contains chemical additives or absorbents
During absorption gases or liquids are channelled over or through, respectively, a particular material, which material may hold the substances contained therein on its surface. The absorbent may be subjected to a thermal regeneration, by means of which the absorbed substances are disengaged from the absorbent, which may again be put to use.
A major criterion for selecting a material suitable as an adsorbent is its active surface area, which makes activated carbon (active surface area of about 300-1000 m2/g) an appropriate material for a number of applications.
However, the use of activated carbon is associated with a number of shortcomings. For instance, due to its inherent flammable nature activated carbon may not be used in combustion gases exhibiting higher temperature simply for safety reasons. Furthermore, thermal desorption is associated with problems, since the temperatures used for this may not be chosen too high, which on the other hand essentially prevents full desorption.
Furthermore, the use of activated carbon may impair the industrial application of fly-ashes accumulating during combustion. These fly-ashes, collected from filters of power plants are generally considered as assets and are employed among others as substitute for cement in the production of concrete. Thus, injecting activated carbon into a stream of combustion gases may worsen the quality of fly-ashes and may compromise their use in the production of concrete, since due to their high specific surface area they tend to absorb concrete aggregates, such as air void formers, which are essential for conveying frost-tau-properties to concrete. For this reason a number of expensive additional measures need to be taken for removing activated carbon from fly ashes.
For solving this problem DE-P 10 2012 012 367 proposes the use of modified Trass, which active surface has been increased to a value according to BET of 40 m2 and above by means of treatment with acids and/or with mixtures of water and tensides. However, a disadvantage resides in that the Trass must be subjected to a pre-treatment prior to its use in power plants or combustion plants, respectively.